CODEWITHSUNDEEP
variableshaskellrecursionioreference
yyny
yyny

Reputation: 1736

Data.IORef vs Recursion

Data.IORef

keepRunning <- newIORef True
let loop = do
    running <- readIORef keepRunning
    if running then
        do
            {- Do stuff -}
            loop running
            return ()
    else
        do
            {- Do stuff -}
            return ()

Recursion

let keepRunning = True
let loop running = if running then
    do
        {- Do stuff -}
        loop running
        return ()
    else do
        {- Do stuff -}
        return ()
loop keepRunning

Is there any reason to prefer one of the above methods above the other? Will the do blocks containing recursion slow the program down compared to outside the do block? If you can replace every instance of Data.IORef with a function, then why does Data.IORef even exist?

Upvotes: 0

Views: 138

Answers (1)

sepp2k
sepp2k

Reputation: 370415

If your {- Do stuff -} in your first version directly assigns keepRunning to the result of some computation, you can of course rewrite it easily to make that computation the condition for the recursion and get rid of the IORef. That would be the preferable solution.

However, if your first version sets keepRunning inside some other function, the equivalent ref-less version could become quite a bit more complicated. So which version is preferable would be less clear in that case. Though you could make the point that such a thing would be bad design in the first place. It's hard to say anything specific without knowing what {- Do stuff -} actually does.

Will the do blocks containing recursion slow the program down compared to outside the do block?

No.

If you can replace every instance of Data.IORef with a function, then why does Data.IORef even exist?

Depending on the APIs you work with, you might not be able to (say you have event handlers in a non-FRP GUI library). Further there are cases where you could remove IORef, but only by hurting performance and/or making code more complicated.

That said your example of using an IORef as a loop condition is one where you could almost certainly write a better version without IORef.

Upvotes: 3

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